Reserves of lignite and subbituminous coals in the United States are estimated to be in excess of 1 trillion tons. Demonstrated reserves are presently placed at 250 billion tons. This vast energy reserve is for the most part located in sparcely populated areas of the western United States and in the western gulf states. A large fraction of the reserves are near to the surface and can be strip mined at low cost.
In 1960, the production of lignite and subbituminous coals was insignificant on the national scale. Presently, these low-rank coals account for approximately 25% of the 900 million tons per year United States production rate. Lignite and subbituminous coal production has accounted for virtually all of the growth in coal production which has taken place in the United States in the last decade.
Low-rank coals have relatively low sulphur content. Low sulphur content generally results in easier compliance with regulations pertaining to emissions of combustion products to the atmosphere. As mined, the sulphur content of lignite ranges from 0.3% by weight to 1% and is most commonly about 0.7%. This compares favorably with bituminous coals which range in sulphur content from 1.2% by weight to 3.5%.
The one significant impediment to the use of lignite and subbituminous coal is their high water content. For lignite mined in the United States the water content ranges from 25% by weight to 45% by weight. High water content supresses the caloric value of the coal, creates handling problems, and increases transportation cost. For these reasons, there is great interest in developing economical methods of drying low-rank coals.
Lignite and subbituminous coals hold water in three ways. First, these low-rank coals hold surface water. The amount of surface water which might be present following a wet cleaning process increases with decreasing particle size. Coals crushed to about 1/4 in. particle size hold between 10 and 15% by weight surface water. When the coal is crushed to 28 mesh size, surface water can account for as much as 45% of the bed weight. Removal of surface water is accomplished quite readily and the process is commonly called dewatering.
Low-rank coals (especially lignite) hold water in interstitial cavities between the carbonaceous fibers. Interstitially held water can account for as much as 45% of the weight of a bed of lignite from which surface water has already been removed. While lignite and subbituminous coals hold comparable amounts of post-wash surface water to coals of similar particle size but high caloric rank, the presence of a large amount of interstitial water is unique to the lower-rank coals. The process used to remove interstitially held water is termed drying. Because interstitial water is held in the small pores between the coal fibers, it is not readily removed by mechanical means. Present thermal drying methods are overly consumptive of energy; consequently, low-rank coals are usually transported and burned without the beneficial effect of drying.
Another way in which water is held in low-ranked coals is in very small closed pores within the carbonaceous fibers. Because the pores are unconnected, removal of this water is extremely difficult and thermal methods provide the only means. For this reason, this water is referred to as bound water. Bound water constitutes only a few percent of the mass of coals of all calorific ranks and as such, does not represent an attractive target for removal, moreover, no economical means exist for its removal.
As mentioned above, the removal of surface water is termed dewatering. For the most part, dewatering is accomplished by mechanical means. Such means include shaker screens which continually disrupt the coal/water matrix and cause the water to drain from the bed under gravity. Centrifuges are also used to dewater low-rank coals. Another means of removing surface water is vacuum filtration. In vacuum filtration, air is sucked through the bed of coal and draws the water with it. Vacuum filtration can be used in conjunction with vibratory and centrifugal methods. Dewatering can be assisted by the addition of surfactants which lower the surface tension of the water and the application of heat, which, by virtue of an increase in temperature, reduces both surface tension and viscosity. The application of an ultrasonic sound field has been shown to assist the dewatering of fine coal particles.
Thermal dewatering methods involve blowing hot air or the products of combustion through the coal bed. Some heat recovery is possible. However, psychrometric restraints make the recovery of the latent heat of vaporization impractical when the steam is mixed with a large quantity of noncondensible gases. For this reason, present thermal dewatering methodologies are relatively uneconomical and are not widely practiced.
Water held interstitially within the particles of low-rank coals makes no contribution to the calorific value of the coal. Rather, from a caloric viewpoint, it is parasitic in that it absorbs heat to achieve evaporation. Moist coal can freeze in stockpiles during cold weather and transportation costs are inflated because unwanted water is shipped with the coal. For these reasons, and because low-rank coals represent one of the nations most important fossile energy resources, there is growing interest in developing means of removing interstitially held water. This process, as mentioned above, is termed drying, as opposed to dewatering which is correctly applied to the removal of surface water. Presently, very little lignite and subbituminous coal is dried in the United States.
Not surprisingly, the techniques which have been applied to the removal of small amounts of water (a few percent by weight) from high-rank coals have been examined for their applicability to drying lignite and subbituminous coals. These methods are based upon the concept of bringing the coal particles into contact with a hot gas stream such as air or the products of combustion. Tumbling the coal particles in a rotating drum through which the hot gas stream is passed and fluidizing a bed of coal particles with the hot gas stream are two methods which have been used to dry high-rank coals. Two factors combine to render these methods unsatisfactory for the drying of low-rank coals. First, the weight of water which must be evaporated per pound of dried product is much greater for low-rank coals than for high-rank coals. Relative to high-rank coals, the overall economics of low-rank coal utilization is much more sensitive to the economics of the drying process. Moreover, the traditional form of drier, described above, does not operate efficiently from a thermal viewpoint. This is because the low partial pressure of the steam in the steam/hot gas exhaust makes it difficult to recuperate the latent heat of vaporization. Consequently, the energy consumed in drying a low-rank coal exceeds 10% of the caloric value of the dried product.
A second reason why existing thermal driers are not suitable for drying lignite and subbituminous coal is that these coals are more susceptible to spontaneous ignition as they approach a fully dried condition than are higher-rank coals. Reactivity in air increases with decreasing rank because low-rank coals contain a higher fraction of volatile matter. Fine particles of dry lignite present a serious explosion hazard in a high temperature environment containing free oxygen.
Another new drier type which is under investigation is the so-called hot water drier. In this drier, lignite is mixed with water to form a slurry which is then heated to about 650.degree. F. at a pressure sufficient to maintain liquid conditions (P.congruent.2200 psi). At high temperature, carboxylic groups within the lignite decompose to form carbon dioxide. The CO.sub.2 gas expels much of the water from the interstitial cavities. Hydrophilic carboxyl groups on the surface of the coal granule are replaced by hydrophobic hydrocarbon groups. This effect along with capillary factors inhibits repenetration of the interstitial cavities by the water as the system is cooled down and depressurized. Test data indicates that this system is capable of reducing the water content of lignite to about 10% by weight. The high operating values of temperature and pressure present significant difficulties in the design of commercial sized equipment.
Although mechanical methods dominate dewatering technology, they yield to thermal methods when it is necessary to do drying which, by our definition, implies removing interstitially held water. Some work on mechanical drying methods has been performed, however. In this regard, it is estimated that a centrifuge operating with a centrifugal acceleration of 5.times.10.sup.4 ft/sec.sup.2 (twice the highest value used in dewatering equipment) will reduce the water content of a 1 ft. deep bed of lignite from 45% to 10% in 1 minute. Unfortunately, this is associated with a hoop stress close to 17000 psi. This stress is close to the allowable operating level for alloy steels.
There is interest in the application of ultrasonic compression waves to assist other mechanical drying means such as centrifuging, vacuum filtration, and pressure displacement. However, this technology is in an embryonic stage and any forecast as to its ultimate role in drying low-rank coal is purely speculative.
It is an object of the present invention to provide an apparatus and method for drying of low-rank coals by removal of interstitial water therefrom in an efficient and safe manner.